1. Field of the Invention
The present invention relates to an adhesive for production of metal bottles. More particularly, the present invention relates to an adhesive for lap-bonding upper and lower members, such consisting of cup-like formed article of a metal, on the peripheral ends thereof.
2. Description of the Prior Art
A bottle-shaped metal vessel formed by lap-bonding upper and lower members, such consisting of a cup-like formed article obtained by draw-forming or draw-ironing of a metallic material, on the circumferential open ends thereof to form a circumferential side seam, is advantageous over a so-called can-shaped metal vessel in various points.
In a conventional packaging metal vessel called "three-piece can", a sealing portion is formed by double-seaming lids to the top and bottom ends of a can body, and in another conventional packaging metal vessel called "two-piece can", a sealing portion is formed by double-seaming a lid to the open end of a cup-like can body obtained by draw-forming or draw-ironing of a metallic material.
In metal vessels having such a double-seam structure, however, the pressure resistance of the sealing portion is insufficient, and saving of the metallic material is a serious problem. More specifically, when a load is imposed on a seam formed by double seaming, the material constituting the seam is first deformed and leakage on the seam or fracture of the seam is readily caused by this deformation under a relatively small load. In order to prevent this leakage or fracture, it is necessary to use a material having a considerably large thickness. In packaging vessels, from the economical viewpoint and in order to reduce the vessel weight, it is always required to reduce the thickness of a metallic material. In the case where the thickness of a vessel wall is reduced, buckling is readily caused by a load applied in the axial direction of a vessel at preparatory steps such as the double seaming step and flanging step.
In case of a metal bottle obtained by lap-bonding upper and lower members, each consisting of a cup-like formed article, on the open ends thereof, even if the thickness of the seam-forming material is very small, deformation of the material is not caused on the seam at all and the seam can resist a load of up to the shear strength irrelevant to the thickness of the material. Furthermore, since the seaming step is not necessary, the thickness of the side wall of the vessel can be reduced without a risk of occurrence of buckling.
However, when a circumferential side seam is formed by lap-bonding upper and lower members on the open ends thereof, various restrictions are imposed on an adhesive used for lap bonding.
In case of a straight side seam of a can body formed by lap bonding, both the ends of the seam can be secured mechanically by double seaming to lids, but the above-mentioned circumferential side seam is not mechanically secured along the entire circumference and dimensional change or deformation of the seam is readily caused. Furthermore, since the diameter of the open end portion is going to change according to changes of the temperature, a stress is readily generated in the adhesive layer. Moreover, the thickness of the open end portion to be formed into a seam is often reduced, and the seam is often deformed by an external force. For these reasons, the adhesive to be used should have a high bonding force to a metallic material or a coating formed thereon, and the adhesive layer should have high mechanical strength and dimension stability and the properties of the adhesive layer should be stably either thermally or with the lapse of time.